Compact positron beam for measurement of transmission moderator efficiencies and positron yields of encapsulated sources

We have constructed a simple positron beam for measuring both the efficiencies of positron moderator foils and the positron yields of encapsulated positron beam sources. The slow-positron emission rate from the moderator is determined from the positron annihilation radiation. This technique is also applied to measure source yields. In addition to the coincidence technique, the source yield is determined by measuring electric currents generated by the annihilating positrons. The performance of the system is demonstrated with different tungsten moderator foils before and after heat treatments, and several sources with primary activities in the range 1-50 mCi.     

The protonation and dimerization of 2,4-, 2,5-, 2,6-and 3,5-dihydroxybenzoic acids in 0.5M sodium perchlorate medium

The protonation equilibria of 2,4-, 2,5-, 2,6- and 3,5-dihydroxybenzoic acids were studied by means of potentiometric titrations at I = 0.5 (NaClO(4)) and 25 degrees . The dimeric species H(6)L(2) and H(5)L(2) were found to form as well as the monomeric species H(p)L in the acidic solutions of 2,4- and 3,5-dihydroxybenzoic acids under the conditions studied. For the other two acids, the protonation scheme can be expressed exclusively in terms of the species H(p)L (p = 1, 2 or 3).     

A positron lifetime and Doppler-broadening study of positron states and free volume in polyethylene

Positron lifetime (LT) and Doppler-broadening (DB) studies of polyethylene have been performed simultaneously in the temperature range between 80 and 300 K. The LT spectra have been analysed assuming four exponential components. Two long-lived components appear, which were attributed too-Ps pick-off annihilation in crystalline regions (₃ = 0.9 to 1.2 ns) and at free-volume holes in the amorphous phase ( to 2.8), The variation in ₄ correponds to an increase of the mean hole size from 0.053 nm³ at 80 K to 0.188 nm³ at 300 K. From the data the glass transition temperature (T _g=195 K), the coefficient of thermal expansion of holes in the glassy and rubbery phase ( _{h, g} = 14.5 · 10^–4 K^–1 and _{h, r} = 189 · 10^–4 K^–1) and the fractional free volume (2.8% to 10.4%) were estimated. The DB curves were fitted by a sum of three Gaussians, the narrowest of which is assumed to represent the self-annihilation ofp-Ps localised at holes. The intensity of the narrow component,I _n, varies between 0 and 7.3% in a similar way as the LT intensityI _4/3 varies. From this it was concluded that other Ps reactions beside pick-off are not important. Further, it was shown that the average positron lifetime is dominated by theo-Ps component,T _{4 g}, while the behaviour of the DB peak height is mainly affected by thep-Ps narrow componentI _n .     

Characterization of the novel ETFE-based membrane

Recently developed ETFE-SA membrane (sulphonated poly(ethylene-alt-tetrafluoroethylene)) has proved to be chemically stable in the direct methanol fuel cell (DMFC). According to methanol permeability measurements, the MeOH permeability through the ETFE-SA membrane is less than 2% of the corresponding value of the Nafion^® membranes. The characterization of the ETFE-SA membranes is done with sophisticated microscopy techniques. The electrochemical inhomogeneity of the membranes is investigated with the scanning electrochemical microscopy (SECM) by mapping proton distribution across the membrane surface. The atomic force microscopy (AFM) is used, when the surface morphology and morphology changes originating from swelling are investigated, while with the scanning electron microscopy (SEM), the composition and the structure of the membranes can be clarified in detail. The sulphur profile along the membrane cross-section gives information about the distribution of sulphonic acid groups and it is detected with the SEM combined with a energy dispersive X-ray spectrometer. Surface hydrophobicity is investigated by water contact angle (CA) measurement. Many remarkable structural differences between different samples are observed during the measurements, e.g. the surface roughness of the ETFE-SA membrane is much higher, when compared to the Nafion^® membranes. Altogether, the surface properties of the ETFE-SA and the Nafion^® membranes are found to differ significantly from each other and the properties of ETFE-SA vary also as a function of manufacturing parameters.